<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
    div{
        width: 200px;
        height: 200px;
        background-color: cadetblue;
        margin-bottom: 10px;
    }
    button{
        margin-left: 10px;
    }
    .change-border{
        border-radius: 50%;
    }
    </style>
</head>
<body>
    <div class="change"></div>
    <button class="btn">变成圆形</button>
    <button class="btn">变成方形</button>
    <script>
        var box = document.getElementsByClassName('change');
        var  btns = document.getElementsByClassName('btn');
        btns[0].onclick = function () {
            box[0].classList.add('change-border')
        };
        btns[1].onclick = function () {
            box[0].classList.remove('change-border')
        };
    </script>
</body>
</html>